A slip control device of a torque converter that uses the engine torque information used with slip lock-up control as an engine torque estimated value is known from the past. This engine torque estimated value is calculated by an engine torque map value being passed through a filter when the dynamic characteristics of the engine are used as the first-order lag of a time constant TED (see Japanese Unexamined Patent Publication No. 2010-270822, for example).
However, the engine has a range for which the engine torque or engine speed are unstable depending on the driving state. In this engine unstable range, when the engine torque is estimated by passing through a first-order lag filter having a time constant TED determined based on the engine dynamics as was done conventionally, the engine torque estimated value fluctuates. Then, overlapping with the slip lock-up control executed following the fluctuating engine torque estimated value, the rotation fluctuates such that there is variation in the engine speed. Due to fluctuation of exhaust noise accompanying this engine rotation fluctuation, there is the problem that this may cause discomfort to the driver.